Polypropylene resins have been widely used in various fields due to relatively desirable price, lightweight and excellent mechanical strength. However, polypropylene resins exhibit unsatisfactory impact strength and rigidity. To overcome this deficiency, a polypropylene resin composition containing ethylene-propylene copolymer rubber (EPM) and talc as an inorganic filler has been proposed.
Conventional wheel covers of automobiles have been produced by press molding with steel. Recently, nylons and polyvinylphenylene oxides having excellent heat resistance have been used for lightening the weight of automotive materials. However, use of these polymers has been a burden due to high cost to use such materials.
Therefore, it has been proposed to use reinforced polyolefin resin compositions such as polypropylene for a low cost. For example, Japanese Laid-open publication no. 6-80839 discloses a wheel cover produced by injection molding of a fiber-reinforced polypropylene resin composition comprising polypropylene, polypropylene having carboxylic acids, silan-treated glass fiber and polyethylene wax. Further, Japanese Laid-open publication no. 10-44701 discloses a wheel cover comprising polyolefin composition made of a crystalline propylene-ethylene block copolymer, an ethylene-propylene copolymer rubber, a polypropylene resin modified with unsaturated carbonic acid or its derivative, talc having less than 10 μm of an average particle diameter, and glass fiber.
Said polypropylene resin compositions for wheel covers use glass fiber to reinforce rigidity and inorganic talc to improve rigidity, impact resistance, and heat resistance. However, the most serious problem in the employment of them with respect to wheel covers is that they have a poor surface appearance quality due to protuberance on the welds.